Rapid pronucleus assembly using cytoplasmic RNAs in fertilized eggs of Xenopus laevis

The size of the nucleus, which serves as the site for essential cellular functions such as replication and transcription, is dynamically altered to support these functions in response to the surrounding environment. During the brief cleavage period in metazoan embryos, the small, hypercondensed sperm nucleus with silenced chromatin undergoes a dramatic transformation into a large, round pronucleus with relaxed chromatin, enabling the activation of chromatin functions necessary for subsequent development. However, it remains unclear whether the egg cytoplasm-specific molecular environment plays a role in pronucleus assembly. In this study, we evaluated the impact of abundant RNAs in eggs on pronucleus assembly by utilizing a cell-free reconstruction of interphase nuclei in Xenopus laevis egg extract. We found that when RNA levels deviated from the conventional concentration, the growth rate of the interphase nucleus decreased. Additionally, the addition of RNAs led to a more dispersed chromatin distribution and the dissociation of sperm-specific nuclear proteins from the chromatin. These chromatin remodeling properties, which were reproducible with the introduction of cationic compounds, facilitated the incorporation of somatic histones into the chromatin in reconstructed nuclei. Based on these findings, we propose that cytoplasmic RNAs promote the rapid decondensation of negatively charged chromatin from a hypercompacted state and the removal of positively charged protamines from sperm chromatin via electrical interactions. This remodeling accelerates pronucleus assembly during the brief cleavage period following fertilization and promotes the rapid growth in nucleus size.